Manufacture of alkali metals

ABSTRACT

There is provided a process for the manufacture of alkali metal by passing an electroyzing current from an anode to a cathode. The anode is in contact with a fused metal halide salt comprising ions of the alkali metal and no other monovalent cations. The cathode is in the form of liquid alkali metal. Interdisposed between the anode and the cathode is a diaphragm. The diaphragm is polycrystalline ceramic material which has ions of the alkali metal or ions capable of being replaced by the alkali metal. The diaphragm is permeable only to monovalent cations and therefore will pass only the cations of the alkali metal which is being manufactured. Halogen can be recovered as the anode product or a halogenated hydrocarbon can be recovered as the anode product by introducing a hydrocarbon or partially halogenated hydrocarbon into the anode compartment.

United States Patent [72] lnventor Anselm Thomas Kuhn Runcorn, England [2]] Appl. No. 711,600 [22] Filed Mar. 8, 1968 [45] Patented Sept. 21, 1971 [7 3 Assignee Imperial Chemical lndustires Limited London, England [32] Priority Mar. 31, 1967 [3 3] Great Britain [3] 14,782/67 [54] MANUFACTURE OF ALKALI METALS 6 Claims, 4 Drawing Figs.

[5 2] US. Cl 204/62, 204/60, 204/68, 204/245, 204/247 [51] Int. Cl C22d 3/06 [50] Field of Search 204/68, 243, 62, 60

[5 6] References Cited UNITED STATES PATENTS 745,958 12/1903 Ewan 204/68 3,017,335 l/l962 Wolfe 204/ 68 Primary ExaminerPatrick P. Garvin Attorney-Cushman, Darby & Cushman ABSTRACT: There is provided a process for the manufacture of alkali metal by passing an electroyzing current from an anode to a cathode. The anode is in contact with a fused metal halide salt comprising ions of the alkali metal and no other monovalent cations. The cathode is in the form of liquid alkali metal. lnterdisposed between the anode and the cathode is a diaphragm The diaphragm is polycrystalline ceramic material which has ions of the alkali metal or ions capable of being replaced by the alkali metal. The diaphragm is permeable only to monovalent cations and therefore will pass only the cations ofthe alkali metal which is being manufactured.

Halogen can be recovered as the anode product or a halogenated hydrocarbon can be recovered as the anode product by introducing a hydrocarbon or partially halogenated hydrocarbon into the anode compartment.

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Attorney:

MANUFACTURE OF ALKALI METALS The present invention relates to the manufacture of alkali metals. More particularly it relates to an improved electrolytic process for the manufacture of alkali metals, especially metallic sodium, and to a novel apparatus for carrying out the improved process.

The most widely used processes for the manufacture of metallic sodium involve the electrolysis of a molten electrolyte containing sodium chloride in admixture with alkaline earth metal chlorides, particularly calcium chloride, These processes have several disadvantages. Because of the need to include alkaline earth metal chloride in the electrolyte in order to depress its melting point to a workable level there is always an equilibrium between free alkaline earth metal at the cathode and alkaline earth metal ions in the electrolyte next to the cathode, so that although sodium is the principal cathode product it is contaminated with alkaline earth metal, for instance calcium. This leads to troubles with blockage of the exit pipes by solid calcium precipitating from the liquid sodium as it cools, to the need to filter the sodium to remove further quantities of calcium impurity and to the need to reprocess the mixture of sodium and calcium together with salt and oxide impurities removed by the filters in order to increase the efficieney of the process. The energy consumption of these prior art processes is high because of the wide gap which is maintained between the electrodes in order to maintain a high working temperature and/or to minimize recombination of the anode and cathode products in the cell. Such recombination as does occur tends to damage the steel gauze diaphragm which is placed between the anodes and the cathode, and this damage is further aggravated by the formation of solid calcium deposits bridging the cathode-diaphragm gap.

The present invention provides a process which avoids all these difficulties and allows the electrolytic production of alkali metal, most usefully sodium, potassium or lithium, in a highly pure state without further treatment and with relatively low energy consumption.

According to the present invention we provide a process for the manufacture of an alkali metal, which comprises passing an electrolyzing current from an anode in contact with a fused metal halide salt comprising ions of the alkali metal and not other monovalent cations, consecutively through the fused salt and through a diaphragm which is a polycrystalline ceramic material permeable only to monovalent cations, to a cathode in the form of the liquid alkali metal and removing alkali metal liberated on the cathode side of the diaphragm.

Suitable polycrystalline ceramic materials for use as the diaphragm are inorganic materials having a crystal lattice structure which contains a large number of defects and containing ions of the alkali metal which it is desired to produce by electrolysis or other monovalent cations capable of replacement by ions of the desired metal. The number of defects must not, however, be so large as to render the material an electronic conductor. One very suitable material is that known as beta-alumina. This has the approximate composition Na 0,llAl and the sodium ions in the crystal lattice can exchange with alkali metal ions, for instance sodium, potassium or lithium ions, introduced from outside. Other suitable materials are certain aluminates, alumino-silicates and titanates.

The invention also includes within its scope an electrolytic cell for the manufacture of an alkali metal which comprises an anode chamber and a cathode chamber separated by a dividing wall, at least part of the area of the dividing wall consisting of a polycrystalline ceramic material permeable only to monovalent cations and any part of the remaining wall being an electrical insulator, the anode chamber being adapted to contain a body of fused metal halide electrolyte in contact with the wall of polycrystalline ceramic material and containing an anode for contacting the fused electrolyte, a liquid alkali metal cathode contacting the wall of polycrystalline ceramic material within the cathode chamber, inlet means for feeding electrolyte to the anode chamber, exit means for removing an anode product from the anode chamber and exit means for removing liquid alkali metal from the cathode chamber.

An electrolytic cell according to the invention may be arranged with a ceramic diaphragm in the form of a flat sheet or alternatively in the form of a closed surface such as a cylindrical wall with one or more anodes inside or outside the cylinder and the liquid alkali metal cathode on the opposite side of the cylindrical wall. When flat sheet diaphragms are employed, the mechanical strength can be increased by incorporating a number of small sheets in a supporting framework to form a window-frame structure so as to build up a large area of diaphragm.

In prior art cells for the manufacture of alkali metals from their halide salts the usable electrolyte compositions have been severely limited because of the need to avoid as far as possible codeposition at the cathode of metals from diluent salts and the need to limit the maximum temperature in the electrolyte to avoid destruction of the alkali metal produced. With the apparatus of the present invention there are no such limitations. Since the diaphragm is permeable only to monovalent cations, the alkali metal is liberated in a high state of purity, for example greater than 99.9 percent, so that a wider range of diluent salts can be used containing cations not permissible in prior art cells and there can be no danger of recombination with halogen liberated on the anode side. Furthermore, electrolyte components of lower purity than heretofore can be employed if desired since the impurities are arrested by the diaphragm. Maintenance of a mixed electrolyte composition is also easier since any diluent salts employed are not lost during electrolysis.

If desired the cell can be worked at high temperature to employ only a halide of the desired alkali metal, for instance sodium chloride, as the molten electrolyte. in general, however,, it is preferable to work at lower temperatures by using lowermelting mixtures of halide salts For the production of sodium, for example, very low-melting mixtures of sodium chloride and aluminum chloride (containing up to 60 moles AlCl may be employed.

With low melting electrolytes the required working temperature may be maintained by surrounding the cell with a hot-air jacket if the resistance-heating effect of the electrolyzing current is insufficient. For starting up the cell or for maintaining the temperature with higher-melting-point electrolytes, electric heaters submerged in the salt or even submerged combustion gas heaters may be employed.

In operating cells according to the invention halogen, for instance chlorine, will usually be removed from the cell as the anode product. However, since these cells can be operated at relatively low temperatures without any danger of alkali metal coming into contact with the anode product, it can readily be arranged to carry out electro-organic reactions, for instance a hydrocarbon halogenation reaction or the further halogenation of a partially-halogenated hydrocarbon, in the anode compartments if desired. Thus, for example, ethylene may be fed into the anode compartment of a cell producing chlorine so that chlorinated ethylenes are formed by reaction between the ethylene and the nascent chlorine and are removed as the anode product.

FIG. 1 and FIG. 2 of the drawings show schematically, and not to scale, sectional elevations of two embodiments of apparatus according to the invention. FIG. 3 shows a plan view of a ceramic diaphragm suitable for use in the cell of FIG. 1. The drawings will be discussed with reference to the production of sodium and chlorine, although as stated hereinbefore the apparatus may be adapted for the production of other alkali metals and other halogens or halogenated organic compounds.

In FIG. 1 a rectangular anode compartment 1 is bounded by sidewalls 2 and the cell cover 3, for example of nickel or bricklined steel, together with a bottom wall 4 forming a diaphragm between the anode compartment and the cathode compartment 5. The diaphragm is preferably constructed as a window-frame arrangement as shown in MG. 3 wherein sheets of beta-alumina 6 are hold in steel frames as indicated at 7. An upper and lower frame (one only shown in plan in FIG. 3) are spaced apart by the beta-alumina sheets so as to preserve electrical isolation between the opposite faces of the diaphragm. Sidewalls 2 and the corresponding walls of the cathode compartment are bolted together with insulated bolts through insulating flanges 8 to maintain electrical isolation between the anode and cathode compartments, the outermost steel frames 7 of the diaphragm being clamped between the flanges and the joints sealed as necessary with fireclay. Graphite anodes 9 suspended on current leads 10 are surrounded by chlorinecollecting domes of nickel 11 provided with offtake pipes 12 for removing chlorine. 13 is an inlet for feeding electrolyte to the anode compartment. 14 and 15 are steel casings surrounding the sidewalls and base of the cell, the space between these casings being filled with heat-insulating brickwork 16. 17 is an empty jacket provided with inlet and output pipes 18 and 19 for circulating a heating medium for instance hot air, through the jacket. Cathode compartment is filled with liquid sodium, which is withdrawn from the cell during elecrolysis by way of pipe 20, which passes through electrically insulating means 21 in walls 14 and I5, and pipe 22, which will normally be connected to a receiver for liquid sodium (not shown). Standpipe 23 and end-caps 24, 25, 26 are provided for ease of filling the cathode compartment with sodium before start-up. The negative current lead is shown at 27 passing through electrically-insulating means 28 in the outer casing to contact the body of sodium metal in the cathode compartment. Since, however, sodium is a good electrical conductor, the negative connection may, if desired be made alternatively to the sodium-filled pipes 20 or 23. This design of cell has a further advantage over prior art sodium cells in that the anodes may be made adjustable in the vertical direction so that they may be moved towards the diaphragm to compensate for anode wear, thereby maintaining a low electrical resistance in the current path through the fused electrolyte.

The cell shown in FIG. 2 is made cylindrical in plan to take advantage of the enhanced strength of a cylindrical-shaped ceramic diaphragm. In this case the cylindrical diaphragm of beta-alumina 29 is connected to a stand-pipe 30 ofcast iron or nickel to act as the negative current connector, and sodium fills this stand-pipe so as to form a good electrical conductor leading to the liquid sodium cathode in the cathode compartment proper. The base of anode compartment 1 is electrically isolated from the cathode compartment by an insulating floor 31. The anodes 32 (only two shown) are graphite rods arranged in a ring around ceramic diaphragm 29. 33 is the outlet for chlorine produced at the anodes. The anode and cathode leads are electrically isolated from each other at the cell cover either by making this of concrete or asbestos board or by passing one or more of the connections through insulating means in the cover. The jackets around the cell are as in FIG. 1 apart from having a cylindrical shape.

The process of the invention is further illustrated by the following example,

EXAMPLE A laboratory-scale elecrolytic cell was constructed in accordance with the schematic representation shown in the accompanying drawing, FIG. 24. An outer glass container 34 was used to form within it an anode chamber 35. A ceramic tube 36 was cemented to the rim of a beta-alumina crucible 37 to form a cathode chamber 38 within the anode chamber. The top of the anode chamber was covered by a glass lid 39 having an upstanding central column 40 spaced around the ceramic tube 36 and closed at the upper end by a gastight seal 4l. A weighted amount of sodium was placed in the crucible 37 as indicated at 42 to form the cell cathode and a steel tube 43 was passed through the seal 41 so as to have its lower end immersed in the sodium and act as the current lead from the sodium cathode. A quantity of electrolyte consisting of equimolar pro ortions of sodium chloride and aluminum chloride was p accd ll'l the anode chamber as indicated at 44 and an anode 45 was passed through sealing means 46 in the glass cover so that its lower end was immersed in the electrolyte. 47 is a thermocouple for measuring the electrolyte temperature. The cell was placed in an oven maintained at 295 C. :5" C. and a stream of dry nitrogen was passed continuously down the tubular cathode current lead as indicated at 48 to flow out of the tube through holes 49 provided near the surface of the sodium cathode 42, over into the anode compartment as indicated by the arrows and finally out of the cell through the exit pipe 50. When the cell temperature had reached equilibrium as indicated by the thermocouple, electrolysis was started by connecting the anode and the cathode lead to a DC source. Chlorine evolved at the anode passed out of the cell with the nitrogen stream through the exit pipe 50. The current was continuously monitored throughout the run, the sodium cathode was then reweighed for calculation of current efficiency and a sample was analyzed. The average current efficiency has calculated at 92 percent and the sodium was found to contain no more aluminum than at the start of electrolysis.

In the claims:

1. A process for the manufacture of an alkali metal, which comprises passing an electroyzing current from 1. an anode in contact with a fused metal halide salt comprising ions of the alkali metal and no other monovalent cations,

2. consecutively through the fused salt and through an electrical insulating diaphragm which is an inorganic polycrystalline ceramic material having ions of said alkali metal or ions capable of replacement by ions ofsaid alkali metal and said ceramic material being permeable only to monovalcnt cations,

3. to a cathode in the form of the liquid alkali metal and 4. removing alkali metal liberated on the cathode side of the diagram.

2. A process according to claim I, wherein the diaphragm consists of beta-alumina.

3. A process according to claim 1 wherein the alkali metal is sodium.

4. A process according to claim 3, wherein the fused metal halide salt is a mixture of the chlorides of sodium and aluminum.

5. A process according to claim 1, wherein halogen is recovered as the anode product of electrolysis.

6. A process according to claim 1, wherein a hydrocarbon or a partially halogenated hydrocarbon is fed into the anode compartment of the electrolysis cell and a further halogenated hydrocarbon is recovered as the anode product. 

2. A process according to claim 1, wherein the diaphragm consists of beta-alumina.
 2. consecutively through the fused salt and through an electrical insulating diaphragm which is an inorganic polycrystalline ceramic material having ions of said alkali metal or ions capable of replacement by ions of said alkali metal and said ceramic material being permeable only to monovalent cations,
 3. to a cathode in the form of the liquid alkali metal aNd
 3. A process according to claim 1 wherein the alkali metal is sodium.
 4. A process according to claim 3, wherein the fused metal halide salt is a mixture of the chlorides of sodium and aluminum.
 4. removing alkali metal liberated on the cathode side of the diagram.
 5. A process according to claim 1, wherein halogen is recovered as the anode product of electrolysis.
 6. A process according to claim 1, wherein a hydrocarbon or a partially halogenated hydrocarbon is fed into the anode compartment of the electrolysis cell and a further halogenated hydrocarbon is recovered as the anode product. 